In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as OSCs in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
Generally there is a need for OSC materials that show high charge carrier mobility. Moreover, for use in OFETs there is a need for OSC materials that allow improved charge injection into the polymer semiconducting layer from the source-drain electrodes. For use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
For use in OPV devices, especially for bulk heterojunction (BHJ) OPV devices, there is a strong need for novel p-type organic semiconductor materials that give improved device performance and do not have the drawbacks of the materials of prior art. The limitations of existing p-type materials relate to deficiencies in light absorption, oxidative stability and charge-carrier mobility. In particular, the new materials should demonstrate the following properties:                low bandgap,        high charge carrier mobility,        being easy to synthesize,        high solubility in organic solvents,        good processability for the device manufacture process,        high oxidative stability        long lifetime in electronic devices.        
One aim of the present invention is to provide new p-type OSC materials, especially for use in BHJ OPV devices, fulfilling the above-mentioned requirements. Another aim is to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing OSC materials as described hereinafter. These OSC materials are based on polymers comprising one or more benzo [1″,2″:4,5;4″,5″:4′,5′]bis(silolo[3,2-b:3′,2′-b]thiophene) units or the corresponding selenophene derivatives thereof, as represented by the following formulae:

(wherein X is S or Se and R1-4 denote for example hydrocarbyl groups)
In prior art the approach to provide low bandgap polymers suitable for application in BHJ photovoltaic devices often is achieved by an increase in the HOMO energy level. The negative implications of this are a potentially higher susceptibility of the polymer to oxidative doping and a loss in the prospective open-circuit voltage (Voc) in a BHJ device, which reduces device efficiency.
In contrast thereto, by using the new polymers as described hereinafter the inventors of the present invention follow an approach that lowers the LUMO energy level in the polymer without affecting the HOMO energy level. Thereby a low bandgap polymer can be obtained without the disadvantages mentioned above. In particular the inclusion of silicon bridging atoms into conjugated species lowers the LUMO energy level.
The polymers of this invention are therefore suitable for use as p-type OSC materials in electronic devices like OFETs and OPV cells, especially in BHJ OPV devices.